Optical systems for imaging lasers or light emitting diodes (LEDs) onto photographic film to reproduce photographs from elecronically stored signals are well known. Such optical systems typically use a polygon mirror scanner which is rotated at a constant angular rate to scan a spot across a film to expose it in successive lines. However, if one focuses light reflected from the polygon mirror scanner with a "classical" lens where the image height is equal to the product of the focal length of the lens and the tangent of the angle the image makes with the optic axis, the spot speeds up as it travels further from the optic axis. This is unsuitable for a print system requiring equally spaced pixels written with data regularly spaced in time. Further, it is unsuitable for a print requiring a constant exposure level across such a scan line. As a result, in such systems, it is desirable to scan a spot across the film at a constant linear rate.
As is known in the art, these problems can be solved using a lens with a predetermined distortion in which the image height is equal to the product of the focal length of the lens and the angle the image makes with the optic axis rather than the tangent of that angle. Such lenses, known in the art as f-theta lenses, solve the problem because the tangent of the angle is always bigger than the angle itself. A further method of solving the above-identified problem is to control the rate of rotation of the polygon mirror scanner.
In addition to the above, there is a further consideration involved in designing an optical scanning system for a printer. That consideration relates to the placement of the objective lens which forms the focus of the lasers or LEDs on the film plane. These are normally classified as either preobjective or postobjective scanning systems depending on whether the polygon mirror scanner precedes or follows the spot-forming optics in the optical path. One is required to use a preobjective scanning optical system for printing on a film which is maintained in a flat plane. This is because the focus of the objective lens is on a curve if the polygon mirror scanner is disposed after the objective lens whereas the focus of the objective lens is in a flat plane if the polygon mirror scanner is disposed before the objective lens.
Notwithstanding the above, the requirement of a preobjective scanning optical system presents further problems in the design of an objective lens for such a system because the objective lens forms the final spot for the entire scan angle. In sum, these problems arise because the objective lens of the preobjective scanning optical system is reuired to: (1) have a flat field; (2) be anastigmatic; and (3) cover a wide field. Moreover, since the scanning mirror of a preobjective scanning system forms the effective aperture stop for the objective lens, the lens has to be designed for a remote stop. As a result of this, as one of ordinary skill in the art will readily appreciate, the objective lens cannot be symmetric about the stop. Consequently, principles of symmetry cannot be exploited to provide a measure of aberration control.
One approach to solving the above-identified problems in accordance with traditional design techniques found in the prior art includes designing the objective lens as an optical system comprising two parts, a collimator and an objective. However, this traditional approach presents a very difficult design problem because the objective has to be corrected for aberrations independent of the collimator. And, the control of these aberrations is more difficult in systems with a remote stop.
As a result, there is a need for a preobjective scanning optical system well-corrected for aberrations for use with an LED or laser printer or photographic system and which can operate with a remote stop. Further, it is preferable for such a system to have a linear scan rate on a plane, a fast relative aperture, and be of low cost and simple construction.